TEMPERATURE/HEAT FLUX ERRORS CUSED BY HIGH TEMPERATURE RESISTIVE SHUNTING ALONG MINERAL- INSULATED, METAL-SHEATHED THERMOCOUPLES

摘要

Commercially manufactured, mineral-insulated, metal-sheathed (MIMS) thermocouples (TCs) have been used at Sandia NationalLaboratories (Sandia) and other experimental laboratories for many years tomeasure temperatures in fires and high-temperature (e.g., 1 200deg C)radiant-heat environments. These TCs are rugged and relatively inexpensive,which make them ideal for large-scale outdoor testing where factors such asweathering and rough handling are important.A particularly insidious source of TC error is thermal shunting of the TCassembly. This error is caused by a sharp drop in the electrical resistivity ofthe mineral insulation at elevated temperatures. When a TC assembly issubjected to high temperature along its length, the lower resistivity of themineral insulation can cause shorting or shunting to occur.This paper describes several cases demonstrating the seriousness of theshunting problem at temperatures previously thought to be immune. Somepreliminary experiments designed to clarify the thermal shunting problemare presented. A model developed under previous work but modifiedspecifically for Type K TCs is discussed. Comparisons of measured andpredicted temperature error data in additional thermal shunting experimentsprovide some degree of confidence in the model's predictive capability.Predictions for several common field test configurations used in Sandiaexperiments are shown in the hopes of sensitizing other researchers in thefield to this problem.